(1) Field of the Invention:
The present invention relates to steel cords to be used as a reinforcement for rubber articles, for example, pneumatic tires to be used in heavy duty vehicles such as trucks or buses and industrial belts. In particular, the present invention is directed to an improvement in fatigue resistance and corrosion propagation resistance of steel cords required for heavy duty tires.
(2) Related Art Statement:
A variety of properties have been required for steel cords to be used in, for example, heavy duty pneumatic tires. Among those properties, adhesion property to rubber is an important property, because the adhesion property affords large effects upon fatigue resistance and corrosion resistance. Regarding improvement upon the adhesion property to rubber, Japanese Utility Model registration publication No. 56-14,396 describes that when core wires are helically formed and a plurality of peripheral filaments are twisted around the outer periphery of the core wire in the state that a plurality of these peripheral filaments do not contact one another, the adhesion property to rubber can be improved together with improvement on corrosion resistance of the steel cord itself through penetrating the rubber up to the interior of the cord.
However, in the case of a two-layer twisted structure having a construction of 3+6 illustrated in Japanese Utility Model registration publication No. 56-14,396, the diameter of the filaments needs be made greater to increase strength of the filaments, which is disadvantageous from the standpoint of fatigue resistance and makes it difficult to penetrate rubber in the interior of the three filament-twisted core wire.
However, since the three-layer twisted steel cords having a construction of 3+9+15 have excellent fatigue resistance, such three-layer twisted cords have been used as reinforcement for heavy duty tires. However, this three-layer twisted steel cord has the problem in that if the diameters of the filaments constituting the cord are made substantially equal, the filaments of each layer contact adjacent ones so that the rubber has difficultly entering the interior of the cord to deteriorate corrosion resistance of the cord.
With the above problem in mind, Japanese patent application Laid-open No. 59-223,503 proposes a 4+9+14 construction in which gaps are retained among filaments by reducing the number of filaments in the intermediate layer and the outer layer from that corresponding to the closest packing structure. However, when the core filaments are helically formed and a plurality of peripheral filaments are twisted around the outer periphery of the core filaments, there is the possibility that the filaments in the intermediate layer fall inside a circle defined by the helically formed core filaments. As a result, the permeation of the rubber into the interior of the cord may be interrupted or the uniform pulling of the filaments in the intermediate layer and the outer layer is deteriorated, resulting in the deterioration in tenacity or fatigue resistance of the cord. In particular, it is difficult to penetrate rubber inside the four filament-twisted core wires. Further, if the number by which the filaments are reduced in the intermediate layer and the outer layer is small, penetrability of the rubber inside the cord is low. If the number of the filaments is further reduced, the tenacity of the cord is lowered to make the cord unsuitable as reinforcement for the tire.